A heating apparatus based on an electromagnetic induction heating (IH: Induction Heating) scheme is designed to cause a magnetic field generated by a magnetic field generation section to act on a heating element, generate an eddy current and cause the heating element to generate Joule heat by this eddy current. This heating apparatus can be used, for example, as a fixing apparatus of an image forming apparatus that causes an unfixed image formed on a recording medium such as transfer paper and an OHP (OverHead Projector) sheet by the image forming section to be heat-fixed.
This fixing apparatus using a heating apparatus based on an electromagnetic induction heating scheme has an advantage of higher heat generation efficiency than a thermal roller type apparatus using a halogen lamp as a heat source and being able to enhance the heat rising speed of a heating element.
Furthermore, a fixing apparatus using a thin-walled heating element made up of thin-walled sleeve or endless belt as the heating element has the heating element of a small thermal capacity, can cause this heating element to be heated in a short time, and can thereby significantly improve rising response before the heating element is heated up to a predetermined temperature.
On the other hand, the fixing apparatus using this type of a heating apparatus takes some safety measures to prevent the heating element from exhibiting runaway effect due to failure of a temperature control system or the like causing fire or producing smoke in a flammable section.
As one such conventional fixing apparatus, a fixing apparatus which disposes a thermostat as an abnormally high temperature detection section which operates upon receiving operating energy through thermal conduction so as to contact a local heat generating part of a heating roller as the heating element and shuts off a current supplied to a circuit that controls the temperature of this heating roller when the surface temperature of the heating roller reaches a predetermined abnormally high temperature using the thermostat is known (for example, see Patent Document 1).
However, in the fixing apparatus disclosed in Patent Document 1, an exciting coil which is the magnetic field generation section and the thermostat are disposed on opposite sides across the heating roller as the heating element, and therefore members for holding the thermostat and exciting coil, wires and terminals are necessary, which causes a problem of increasing the number of parts and assembling man-hours and also the area occupied by the apparatus.
Furthermore, in the case of the fixing apparatus disclosed in Patent Document 1, if its heating roller is made of a magnetic member, when the temperature of the heating roller exceeds its Curie temperature, magnetic permeability of the magnetic member of the heating roller decreases drastically and magnetic flux leaks from the heating roller. This leakage flux is induced to the magnetic member around the heating roller and causes the part of the heating roller facing this magnetic member to locally generate high heat. For this reason, in this fixing apparatus, when high heat is locally generated in any parts other than the location of the thermostat as described above, the fixing apparatus itself may be damaged or catch fire before the thermostat operates. Especially, when the rotation of the heating roller is stopped, there is a problem that even if high heat is generated locally in any part other than the location of the thermostat, the thermostat does not operate.
As a heating apparatus to solve the above described problem caused by the temperature of the heating roller exceeding its Curie temperature, a heating apparatus comprising a thermo switch as an abnormally high temperature detection section disposed at a position facing an exciting coil as the magnetic field generation section with a heating member as the heating element interposed in between, and further a leakage flux induction member disposed at or near the position of the thermo switch and made up of a magnetic member that induces leakage flux from the heat generating layer generated when the temperature of the heat generating layer of the heating member exceeds the Curie temperature of the magnetic member of the heat generating layer is known (for example, see Patent Document 2).
In the case of the heating apparatus according to this Patent Document 2, when its temperature adjustment/control system does not operate normally due to a failure of the apparatus or the like and its excessive power supply to the exciting coil continues, the temperature due to heat generation of the heating member increases. At this time, when the temperature of the heat generating layer of the heating member exceeds the Curie temperature of the magnetic member used for the heat generating layer, the magnetic permeability of the heat generating layer decreases drastically and magnetic flux which has formed a magnetic path in the heat generating layer leaks. Most of this leakage flux is induced to the leakage flux induction member. This causes the magnetic flux in the heat generating layer of the heating member at a position facing the leakage flux induction member to increase relatively compared to other parts and causes the temperature of the heating member to increase locally in this part, causing the thermo switch to operate earlier.
For this reason, in the heating apparatus disclosed in Patent Document 2, when runaway effect is produced due to a failure in its temperature control system and the temperature of the heat generating layer of the heating member reaches an abnormally high temperature which exceeds the Curie temperature of the electrically conductive magnetic member making up the heat generating layer, this allows the thermo switch which is a thermosensitive safety apparatus to operate earlier in order to shut off a power supply to the heating apparatus.    Patent Document 1: Japanese Patent Application Laid-Open No. HEI7-319312    Patent Document 2: Japanese Patent Application Laid-Open No. 2001-267050